Internal combustion engines use a combustion cycle to combust fuel and oxygen to convert the energy of the fuels into mechanical energy for powering devices such as automobiles, locomotives, generators, and many other devices.
Many modern engines (spark or compression ignition) utilize the exhaust gases expelled from the engine for various beneficial activities. One particular activity is the use of the exhaust gases to power a turbocharger. The flow of the exhaust gases drives a turbine of the turbocharger, which in turn drives a compressor to increase the amount of air supplied to the engine. The increased air to the engine allows higher power density, improving fuel efficiency as well as transient responsiveness.
Exhaust gases are also used to reduce emissions. One problem with internal combustion engines, and particularly diesel engines, is that at high combustion temperatures, NOx is generated. Further, the production of NOx is typically non-linear such that an incremental increase in temperature can significantly increase the rate of NOx production.
Another problem with internal combustion engines, and particularly diesel engines, is that when the combustion cycle operates relatively fuel rich, i.e. close to the stoichiometric ratio for complete combustion (but still with an excess of oxygen) the engine will produce a large amount of soot. The soot can inhibit the operation of the engine, downstream components of the exhaust system, as well as provide undesirable emissions.
With ever increasing energy/fuel costs as well as ever more stringent emission reduction regulations, it is desirable to increase the fuel efficiency while decreasing the amount of emissions generated.
To reduce NOx emissions, exhaust gas is recirculated, which is known as exhaust gas recirculation (“EGR”, which will also be used to refer to the actual exhaust gas that is being recirculated), to reduce the in-cylinder temperatures during combustion. The primary components of EGR are carbon dioxide (“CO2”) and water vapor, plus nitrogen and leftover oxygen that wasn't consumed by the fuel in a previous cycle. In general, the exhaust gas is substantially inert. Carbon dioxide and water vapor have high specific heats relative to air. As such, it takes a larger amount of energy to raise the temperature of these components. Therefore, in-cylinder temperatures during combustion will be reduced with these gases present as compared to if they are not present. EGR can be used to reduce in-cylinder temperatures.
To improve the EGR's ability to control or limit in-cylinder temperature, the EGR is usually passed through an EGR cooler that removes heat energy from the EGR prior to mixing the EGR with the intake gas. This reduces the temperature of the EGR allowing the EGR to absorb more energy during combustion to more effectively control and maintain in-cylinder temperature.
A schematic illustration of a standard, four-stroke internal combustion engine 10 is illustrated in FIG. 1. As illustrated, the engine 10 includes a single exhaust manifold 12 coupled to all of the exhaust valves 14 of engine 10. The single exhaust manifold 12 collects all of the exhaust gases generated by the engine 10. A turbo flow path 15 couples the exhaust manifold 12 to the turbine 16 of the turbocharger 18. An EGR flow path 20 operably couples the exhaust manifold 12 to the intake flow path 22 to mix the EGR with the intake gases.
One problem with this current arrangement is that all the engine exhaust gas is fed into a single exhaust manifold, which is connected to both the EGR pathway and the turbine inlet pathway. The problem is that each of these pathways has separate requirements for optimal conditions: (a) the EGR pathway wants low temperature exhaust gases with just sufficient pressure to overcome the intake manifold pressure and drive just sufficient EGR flow from the exhaust manifold to the intake manifold, while (b) the turbine wants high temperature, high pressure and high mass flow to improve the turbocharger output and create the pressure in the intake manifold via turbocharging. Currently, the two functions are combined by use of the common exhaust manifold which leads to a compromise. The EGR loop is too hot and requires high pressure to drive the EGR, while the turbine loop is too cool and the pressure too low as the EGR loop has utilized some of the available pressure. The need for independence is driven home by the recognition that the turbine loop creates the boost pressure that the EGR loop must overcome and that extra temperature in the EGR loop must be removed via a heat exchanger. Once these are recognized, it is clear that it is very unlikely that a single exhaust valve timing to a common manifold will serve both EGR needs and turbine needs well.
Embodiments of the present invention provide improvements over the current state of the art to improve the distribution of the exhaust gases to the turbine as well as for use as EGR to improve fuel efficiency while reducing emissions. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.